John makes some good points above. While it doesn't help with your original question, if you want to maximize your sales price, good record keeping and keeping up with maintenance is key.
Buyers are generally impressed by and will generally pay more for a vehicle (trailer or otherwise) that has a well-documented history and maintenance records. Have a log book and record everything. Save the receipts, etc.
This will help you substantiate your asking price and differentiate you from other potential sellers who may ask for less or be willing to sell for less than you. Being able to respond, "Oh, that one is $10,000 less? What is its maintenance history? Here's mine" can be helpful when you want to explain why yours is $10,000 more.
I recommend a simple Moleskin book for vehicle logs. Something like this:
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Keep one for maintenance records and one for travel records (where you went, how long you were there, etc).
Whenever you get home from a trip, or do (or have done) work on the trailer, transfer your receipts from the Moleskin logbook to a Manila envelope with your master records.
At the end of the day, the "fair" price is what a buyer and seller agree on. No surprises there. Good record keeping can help swing the transaction to your favor.